gdb: Remove OpenBSD/m88k support

gdb: Remove OpenBSD/m88k support

Support for m88k was fully removed from bfd, which broke gdb
--enable-targets=all builds:

  > gdb/m88k-tdep.c: In function void _initialize_m88k_tdep():
  > gdb/m88k-tdep.c:867:21: error: bfd_arch_m88k was not declared in this scope
  >    gdbarch_register (bfd_arch_m88k, m88k_gdbarch_init, NULL);

There's no point in keeping GDB support for OpenBSD/m88k with no bfd
support, so this commit simply removes the port.

gdb/ChangeLog:
2018-04-16  Pedro Alves  <palves@redhat.com>

	* MAINTAINERS: Remove m88k.
	* Makefile.in (ALL_TARGET_OBS): Remove m88k-tdep.o.
	(HFILES_NO_SRCDIR): Remove m88k-tdep.h.
	(ALLDEPFILES): Remove m88k-bsd-nat.c and m88k-tdep.c.
	* NEWS: Mention that support for OpenBSD/m88k was removed.
	* configure.host (m88*-*-*): Remove support.
	* configure.nat (m88k-*-*): Remove support.
	* configure.tgt (m88*-*-openbsd*): Remove.
	* m88k-bsd-nat.c, m88k-tdep.c, m88k-tdep.h: Delete.